1. Field of the Invention
The invention relates to a testing method for determining the quality of an electrode paste used for forming a secondary battery and to a method of manufacturing an electrode for the secondary battery, the method being related to the testing.
2. Description of Related Art
An electrode of a secondary battery includes a current collector which is composed of a metal material having good conductivity and an active material layer which is formed on the current collector. In a typical method for forming the active material layer, first, a primary kneaded material is prepared by kneading an active material and a thickener in an aqueous solvent. A binder is then added to the primary kneaded material, the resultant is further kneaded, and in this way, a composition in the form of a paste (an electrode paste) (the composition may also be a composition in the form of slurry or a composition in the form of ink; the same will be applied hereinafter) is prepared. A surface of a current collector is coated with the electrode paste obtained as described above, the electrode paste is then dried, and in this way, an active material layer is formed.
When the electrode paste is kneaded, the active material particles collide with each other in the solvent, and accordingly, the active material particles may undergo cracking, flaking, chipping, or the like. In such cases, the number of reactive sites on the surfaces of the active material particles is increased, and therefore thermal stability or durability is reduced in some cases. That is, the change in the properties of the electrode paste leads to a qualitative variation of the electrode and can consequently exert an influence on the battery performance. Accordingly, in order to stably manufacture a secondary battery that has good battery performance, a more homogenous electrode paste needs to be used. As a technique relating to this requirement, for example, Japanese Patent Application Publication No. 2010-272287 (JP 2010-272287 A) discloses a method of testing a negative electrode paste of a lithium secondary battery, in which the primary kneaded material as described above is subjected to centrifugation, the supernatant is collected, and from the results of spectral analysis of the supernatant, the quality of the primary kneaded material is evaluated (determined).
However, when a continuous kneading device such as an extrusion kneading device (extrusion kneader) is adopted, due to the constraints of facilities, it is difficult to collect the primary kneaded material, and the method in JP 2010-272287 A cannot be adopted. Furthermore, according to the examination conducted by the inventor, for example, in a case of using a water dispersible binder, when the electrode paste to which the binder has been added is tested, the quality of the electrode paste cannot be accurately determined with the method in JP 2010-272287 A.